There has been known a type of roll crusher, as shown in FIGS. 5 and 6, in which a pair of rolls 2 and 3 respectively facing each other and rotating in opposite direction to each other is provided, feed material such as rocks and ores to be crushed is supplied through the supply port 5 into the crushing chamber 6, that is, a space formed in between the pair of rolls, and the feed material supplied is crushed by compression while being rolled with said pair of rolls 2 and 3.
The type of roll crusher has a crushing chamber 6 (a region indicated by chain line) as shown in FIGS. 7a and 7b, whose longitudinal side faces 6a and 6b are formed respectively by the outer surfaces of the pair of rolls 2 and 3, and whose end faces 6c and 6d coincide with the openings formed in between the end faces 2a and 2b as well as 3a and 3b of said pair of respective rolls 2 and 3. But the crushing chamber shown is an example for explanation, therefore not necessarily limited to the shape shown, but varying depending on the crushing conditions.
On the other hand, some roll crushers according to the prior art are provided with side plates called cheek plates to prevent crushed stock from flowing out from the end openings 6c and 6d of the crushing chamber 6. During the process of crushing by the rolls 2 and 3, this type of roll crusher has no capability sufficient to prevent material being crushed from being pushed out of the crushing chamber 6 through the lower end portions of the end openings 6c and 6d (higher pressure applied on material to be crushed here), thus resulting in higher pressure applied on the rolls 2 and 3 at the roll center, and in lower pressure at both ends.
Repeated crushing with such different pressures distributed on the rollers may cause partial wear of the rolls 2 and 3, as shown in FIG. 8, thus resulting in a uniform shape with the smaller middle section and the larger end sections. Due to such partial wear a constant axial crushing clearance between rolls is not maintained. Therefore, in crushing material with a relatively small clearance in such case as making crushed sand, crushing clearance at the middle section is too large, although the rolls come into a close contact with each other with zero clearance at both ends. This partial wear of rolls has been long well known as the worst defect of the roll crusher, which causes a failure of effective crushing, thus necessitating laborious repair work to abrade the roll surface to restore a uniform axial crushing clearance between rolls.
Heretofore, in crushing rocks or ores by means of a roll crusher, to have a large crushing ratio, roll clearance is adjusted to be equal to or smaller than the particle size of desired products. Particularly for fine particle products, to have a large fraction of fine particles in crushed products, it was common for roll clearance to be adjusted to about 1/2 particle size of desired products. Crushing mechanism according to the prior art may be described as follows, referring to FIG. 14. A clearance between a pair of opposing rolls 2 and 3, that is, crushing clearance S is smaller than particle diameter F of feed material to be crushed, and equal to or smaller than the particle diameter P of desired products. Particles of material to be crushed are subjected to a continuously increasing compressive load and are eventually broken from the time when they come into contact with the surfaces of the pair of the opposing rolls to the time when they pass between the closest positions of the two opposing rolls.
As stated above, the roll crusher according to the prior art has a small crushing clearance S, thus limiting the throughput capacity of feed material through the crushing chamber, resulting in a low productivity of products. Especially, the smaller the particle size of desirable products, the smaller the crushing clearance, thus further restricting the productivity.
And, because feed material to be crushed is pressed by the roll 2 and 3 from the left and right sides of the drawing, the size and shape of broken particles are regulated as regards the horizontal direction, but no regulation can be expected as regards other two directions such as vertical and perpendicular to the paper surface of the drawing. Therefore, products according to the prior art include a large fraction of particles having sizes larger than the crushing clearance S, and it is well known that they contain a lot of flat or slender particles.